Optical absorption spectroscopy systems and methods for use in downhole conditions are known and have been utilized in the oil industry for sensing physical and chemical properties of in-situ fluids. In particular, such logging tools are used for qualitative, e.g., contamination monitoring, or quantitative analysis, e.g., gas-oil ratio. Such methods include a source of light, a sampling cell, and a spectrometer located downhole.
Downhole environmental conditions, such as high temperature and pressure, impose constraints on the components involved in such methods, such as light sources and spectrometers. For example, a light source that can function in these harsh conditions is limited to a black-body type. Therefore, advantages may be achieved by removing both light source and processing aspects of optical spectrometers from downhole and locating them at the surface. A source of light located at the surface may be used to direct light through a single optical fiber. Such light may be modulated in the same manner as electrical signals and used to carry information as to the measured or detected characteristics of interest. The single fiber can also be used to retrieve light carrying such information from downhole to the surface where it can be processed through state-of-the-art instruments that can perform a spectral analysis.
In the past, use of optical fiber spectroscopy within a wellhole was disclosed in U.S. Pat. No. 6,437,326, of common assignment to Schlumberger Technology (“Schlumberger”) with the present invention. The disclosure of this Schlumberger '326 patent is hereby incorporated by reference as though set forth at length.
As noted in the Schlumberger '326 patent, for spectroscopic applications, where it is important to have accurate determinations of the absorption of light in the sample itself, attenuation of the light traversing the optical fibers in the system must be considered and corrected. The '326 patent discloses use of a two-optical-fiber configuration, or a single-fiber configuration with Time Division Multiplexing (TDM) in order to compensate for this attenuation.
In a two-fiber configuration, one fiber is used for sample measurements and the other fiber is used as a reference to compensate for the attenuation in the system outside of the sample cell. However, in practice, the light paths for the sample and reference have different lengths, and such a configuration, therefore, leads to inaccuracies in the compensation for cable attenuation, which, in turn, adversely impacts the accuracy of the sample measurements. Additionally, a double-fiber configuration requires twice the length of fiber as a single-fiber configuration. In wells, which may typically involve depths of 25,000 feet or more, doubling the length of optical fiber substantially increases the overall cost of an operating system.
A TDM-based system requires a pulsed light source, which inherently reduces the optical power available for downhole system applications. Additionally, in systems where multi-sampling cells are employed, the demodulation of the light beams becomes a necessary but cumbersome step in processing optical signals to extract the measurement data. Although the above referenced Schlumberger '326 patent system is a significant advance in the art, room for worthwhile improvement remains.